A conventional optical filter device is described on pages 123 to 126 of "OPTICS COMMUNICATION, Volume 10, number 2, Feb. 1974" as entitled "Analysis of the transmission, reflection and noise properties of distributed feedback laser amplifiers". The optical filter device comprises a distributed feedback structure having a diffraction grating which is advantageous in regard to a wavelength selection and an optically integrating property as compared to a Fabry-Perot resonator having cleaved facets.
Such an optical filter device is one of the important devices which is widely applied to an optical communication apparatus, an optical switching system, an optical information processing apparatus and so on to select a predetermined wavelength of an optical signal from optical wavelength multiplexed signals. In each application, the optical filter device is required to have a satisfactory property of a wavelength selection, and a large wavelength tuning range. The optical filter device is further desired to be a transmitting type of a wavelength selection filter device, through which a predetermined wavelength of an optical signal is passed because it is inevitably to be included in an optical integrated circuit.
In regard to the aforementioned transmitting type of a wavelength selection filter device, several devices have been studied. Among them, an optical filter device which comprises an optical feedback structure having a wavelength selection property in an optical amplifying device utilizing a semiconductor active layer is expected to be put into a practical use because the tuning of the wavelength selected is varied dependent on the density of carriers injected into the active layer and it is suited to be included in an optical integrated circuit. Especially, an optical filter device which comprises a distributed feedback structure having a diffraction grating as described in the aforementioned "OPTICS COMMUNICATIONS" is advantageous in regard to wavelength selection and an optically integrating property as explained before.
According to the conventional optical filter device, however, there is a disadvantage that an optical gain for a selected wavelength and an intensity of spontaneous emission light are changed in a case where a density of carriers injected into the active layer is adjusted for the turning of selected wavelength. As a result, a density of carriers injected into the active layer is limited to be adjusted in principle in obtaining predetermined properties of a wavelength selection and a ratio of signal to noise. For this reason, a turning range of a wavelength selection is as narrow as several .ANG. so that the optical filter device is only used for selecting a predetermined wavelength of an optical signal from wavelength multiplexed signals of several channels.